Obstructive Sleep Apnea (OSA) is a common disorder in western society, affecting between approximately 4 to 9% of the general population over the age of 40. It is a condition where the upper airway may be occasionally obstructed, either partially or completely, during sleep. Such obstructions may result in an interruption of sleep or at the least diminished quality of sleep. The primary clinical symptom is daytime hypersomnolence. This condition can significantly interfere with a patient's ability to function normally. Long-term medical consequences of chronic, untreated OSA may include pulmonary and systemic hypertension, cardiac arrhythmias, increased likelihood of myocardial infarction and ultimately, cardiac failure.
To treat obstructive sleep apnea, upper airway collapse can be relieved in many ways. One approach is to bypass the upper airway so that even if the airway collapses, there is an alternative route for air to flow. Such a bypass is accomplished through a tracheostomy procedure. This of course is highly invasive, costly and not currently favored. Another approach is to reverse the upper airway collapse. Many treatments may be used to reverse the upper airway collapse, including weight loss, pharmacological management, upper airway reconstructive surgery, or continuous positive airway pressure (CPAP). CPAP at present is now the most favored method for treating OSA, being used in approximately 80% of all newly diagnosed cases of OSA. In spite of its current widespread use CPAP is still not the ideal treatment. For example less than half of CPAP patients use CPAP regularly. More conservative measures such as weight loss and pharmacological treatment have also met with minimal success due to compliance problems or the development of side effects. Surgical reconstruction of the upper airway (uvulopalatopharyngoplasty or UPPP) has also met with equivocal results, mostly due to an inability to select the optimal patient for this particular form of treatment.
Stimulation of the upper airway and in particular of the hypoglossal nerve in synchrony with the inspiratory phase of respiration is a further alternative therapy for patients with OSA. Patients treated with such a upper airway stimulation system are provided the opportunity to gain restful, uninterrupted sleep otherwise not possible due to the obstructive apnea episodes. At present such a system is available from Medtronic, Inc. The system for stimulation consists of an implanted programmable pulse generator, such as the Medtronic Inspire Model 3024 Implantable pulse generator, a stimulating lead, such as the Medtronic Model 3990 half cuff electrode, and a dP/dt pressure sensing lead to signal respiration, such as the Medtronic model 4322 pressure sensor. Preliminary results demonstrate that hypoglossal nerve stimulation for treatment of OSA is successful.
In spite of the initial success, stimulation synchronized with respiration is, in some patients, a problem due to cardiac artifact in the pressure signal. Although in some patients the pressure signal is only minimally affected by the cardiac artifact, resulting in excellent synchronized pacing, in other patients cardiac artifact makes detection of respiration less reliable.
FIGS. A and B illustrate the problem of cardiac artifacts in the pressure signal. FIG. A details the sensed pressure signal due to respiration. As seen in this embodiment, the sensed pressure signal is generally sinusoidal. As further seen in this figure, the bottom tracing shows the stimulus markers, positive spikes indicating stimulation begins while negative spikes indicate the stimulation stops.
FIG. B, in contrast, shows a pressure signal having an unacceptably large cardiac artifact. As seen, this type of pressure signal is much less rhythmic or repetitive as that shown above. As can be appreciated, this non-rhythmic signal makes the interpretation of the pressure signal much more difficult, if not impossible. Thus, when such a large degree of cardiac artifact is present a reliable and accurate sensing of pressure is inhibited, leading to a not optimally therapeutic delivery of stimulation, seen here as lower tracing in this figure.
Thoracic impedance measures is a widely accepted method of detection of respiration, the concept being a change in lung volume during respiration. However, during an obstruction of the upper airway, the lung volume hardly changes, making thoracic impedance measures unsuitable for detection of respiratory effort. Patients with obstructive sleep apnea still try to breathe and, therefore, lower the diaphragm. Therefore, impedance measures over the diaphragm should result in a signal related to respiratory effort. This signal can be used as an input signal for a hypoglossal nerve stimulator.